A method of forming a direct positive image is well known, where a direct positive photographic material having a previously non-fogged internal latent image-type silver halide emulsion is imagewise exposed and then subjected to surface-development after or during fogging.
The above-mentioned latent image-type silver halide photographic emulsion means a silver halide photographic emulsion of such a type that the silver halide grains therein have light-sensitive nuclei essentially in the inside thereof and a latent image is formed essentially in the inside of the grains by exposure.
Various techniques have been known in this technical field. For instance, U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577 and British Patents 1,151,363, 1,150,553 and 1,011,062 illustrate the essential techniques of the field.
Using the known methods, direct positive photographic materials having a relatively high sensitivity can be obtained.
The details of the mechanism of forming direct positive images are set forth, for example, in T. H. James, The Theory of the Photographic Process, Ed. 4, Chap. 7, pages 182 to 193 and U.S. Pat. No. 3,761,276. JP-A-1-52146 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") illustrates a technique of incorporating a metal into silver halide grains so as to elevate the contrast of the photographic material containing the grains. JP-A-1-254946 illustrates a method of obtaining a hard (high-contrast) direct positive photographic material, which contains a thiosulfonic acid in the emulsion and which forms an image having a lowered Dmin.
In order to put a direct positive photographic material into practical use, the material must provide a hard image having a high Dmax and a low Dmin. However, where additives are added to conventional direct positive photographic materials so as to lower the value of Dmin of the image to be formed, the value of Dmax is often lowered because of the addition. For instance, in the method of the above-mentioned JP-A-1 52146, Dmax of the image formed is often lowered when the material contains a large amount of a metal.
In accordance with the method of JP-A-1-254946, a direct positive photographic material providing a hard image having a lowered Dmin can be obtained. However, the material to be produced by the method has drawbacks in that (1) the fresh sensitivity is low and (2) the time-dependent variation of sensitivity during storage (or increase of sensitivity) is great.